Solar systems are units which generally comprise a solar absorber for the absorption of solar heat and, by means of a heat transfer medium, convey the absorbed heat from the solar absorber to the heat exchanger of a hot water system. Vacuum tube collectors are increasingly being used for the effective exploitation of solar energy. The good thermal insulation makes it possible to achieve not only a high efficiency but also a very high no-load temperature which can be up to +260.degree. C. when the unit is shut down and no heat is being taken off. Heat transfer media used are customarily glycols such as ethylene glycol but preferably the nontoxic 1,2-propylene glycol together with appropriate corrosion inhibitors.
Temperatures above +220.degree. C. result, even under pressure (customarily 2-4 bar), in vaporization of the water and later the glycols when the units are in a no-load condition. This can result in crystallization of the corrosion inhibitors. The salt-like slurry formed with the residues of glycol can deposit, in particular, in the thin tubes of the vacuum collectors and lead to the tubes becoming blocked by decomposition products. These deposits can be so tenacious that they no longer dissolve in fresh solar fluid, nor even in water. The consequence can be failure of the unit, which leads to expensive replacement of the tubes concerned.
This problem has already been recognized in DE-A-195 25 090 and attempts have been made to solve it. An addition of 10-40%, preferably 15-25%, of a relatively high molecular weight polyalkylene glycol having a molar mass in the range 195-400 was proposed, the remainder being 1,2-propylene glycol, ethylene glycol or a mixture of these two compounds. This basic mix is mixed with water in a ratio of 40-70% by weight in order to achieve the necessary antifreeze and heat transfer properties.
However, it has been found that the heat transfer media of the prior art do not provide a satisfactory solution to the problem of overheating of solar units. Decomposition of the constituents still occurs, with the consequences described above.
It is an object of the invention to find a heat transfer medium whose tendency to boil and decompose is lower still than the heat transfer media of the prior art.